Serum IgG Response to Endotoxin and Outer Membrane components of Actinobacillus actinomycetemcomitans in Localized Juvenile Periodontitis Patients Actinobacillus actinomycetemcomitans is the primary periodontal pathogen in localized juvenile periodontitis (LJP). Recent evidence has suggested that in patients with refractory periodontitis the primary humoral response is directed against the endotoxin produced by this organism. Specifically, IgG antibodies are produced against polysaccharide moiety of the LPS in Aa. However, the IgG generated is principally of the IgG2 subclass. This subclass does not generate opsonic antibody which leads to a protective immune response. One of the long term objective is to develop a conjugated vaccine for LJP patients that would help generate protective type of antibodies that the patient could use for protection against Aa and thereby prevent infection. In addition to an altered humoral response in LJP patients to endotoxin, there also appears to be an elevated IgG2 subclass response to a 29 kDa major outer membrane protein (OMP). Current efforts have been made toward molecular cloning of this 29 KDa OMP to further elucidate the nature of this altered IgG2 subclass response. Key words: IgG subclass response: LPS; Outermembrance proteins